You are absolutely right. I also try to support 5.6.x for some of my modules. (I dropped 5.005_04 not too long ago for Text::CSV_XS). What you will see however, is that some modules will require new TESTING features, like Test::More's done_testing and subtests. These might require new(er) build tools and the snowball starts rolling.

I never stated that 5.6 would not be able to install new CPAN stuff, but that one should be aware that it will/might not work out of the box for many CPAN updates in the (near) future.